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That is what you asked. The gestation period is the time it takes for the young to mature inside the fish prior to giving birth. Note when you first noticed the guppys pregnant appearance and do the math... No one here can say "Oh 2 days..". When you see the birth canal widen, then you have between 2 and 24 hours and depending on the age and fertilization of the female you could have 1 - 25 babies.